1. Field:
The invention is in the field of methods and apparatus for measuring the extent of gas accumulation in the body of a scuba diver.
2. State of the Art:
A column of fresh water 33 feet high or sea water 32 feet high exerts one atmosphere of pressure. Since the water surface is already at one atmosphere pressure, the pressure at a depth of 33 feet in fresh water is 2 atmospheres. The pressure increases by 1 atmosphere for every 33 feet further that one descends. Thus, at a depth of 100 feet, the pressure exerted on a diver will be 4 atmospheres.
Air is made up primarily of oxygen (about 20%) and nitrogen (about 80%). At sea level, approximately one liter of nitrogen is dissolved in an average person's body. Nitrogen is about five times as soluble in fat as in water so that more than half of the nitrogen is dissolved in body fats, even though the fats only make up about 15% of the body. Nitrogen is not metabolized by the body, so it remains dissolved in the body to an extent dependent on the pressure. Oxygen is metabolized and thus is not a problem when a diver breathes compressed air.
As a diver descends, the pressure on his body increases and the pressure of the compressed air which he breathes must also increase in order for him to breathe properly. As the pressure increases, the amount of nitrogen dissolved in a diver's body also increases. Thus, at 33 feet a diver will have two liters of nitrogen dissolved in his body; at 100 feet his body will hold four liters. However, the nitrogen is not dissolved instantly. Rather, several hours are required before the body becomes saturated with nitrogen as all of the tissues in the body come into equilibrium with the gas pressure in the diver's lungs. Furthermore, the nitrogen dissolves at different rates in different parts of the diver's body. The water in the diver's body is saturated in about one hour, whereas the fat, which requires much more nitrogen before it is saturated, and also has a poor blood supply to carry the nitrogen, reaches saturation only after several hours.
Thus, the deeper a diver descends and the longer he remains submerged, the more nitrogen will dissolve in his body. As a diver ascends, the pressure on his body will decrease and the nitrogen will be liberated from his tissues. If the ascent is too rapid, actual bubbles of nitrogen will form. Bubbles forming in the brain, spinal cord, or peripheral nerves can cause paralysis or convulsions, or other effects. Bubbles in the joints or muscles cause severe pain. Nitrogen bubbles in the respiratory system can cause difficulty in breathing and heavy coughing. In any event, the experience is painful and may result in permanent injury. To avoid these effects a diver must ascend slowly enough to allow the excess nitrogen to be expelled slowly from his body without bubble formation, or he must ascend before too much nitrogen has dissolved in his body. The United States Navy has established maximum safe limits for diving at various depths without requiring decompression procedures. For instance, it has been determined that a diver can ascend immediately to the surface without danger of nitrogen bubble formation in his body if he remains no longer than 100 minutes at a depth of 50 feet, or 25 minutes at 100 feet, or 5 minutes at 150 feet.
Since the amount of nitrogen dissolved in the body is a function of depth and time, it would be advantageous to have an instrument which could take into account the depths to which a diver dives and the length of time he spends at these various depths, and gives a readout or indication of when the diver must ascend to avoid going through decompression procedures. Such instruments, called time-depth integrators, are available commercially, but the present instruments are mechanical devices subject to shock and wear, and are expensive.